Various arrangements and configurations of prior art slip-rings employing FOT brush assemblies are representatively shown and described in U.S. Pat. No. 7,105,983 B2, U.S. Pat. No. 7,339,302 B2, U.S. Pat. No. 7,423,359 B2, U.S. Pat. No. 7,495,366 B2 and U.S. Pat. No. 7,545,073 B2. These prior art references are assigned to the assignee of the present application, and are hereby incorporated by reference.
Electrical contacts are used to transfer electrical power and/or signal(s) between a rotor and a stator. These devices are used in many different military and commercial applications, such as solar array drive mechanisms, aircraft and missile guidance platforms, wind energy systems, computed tomography (CT scan) systems, and the like. In some of these applications, slip-rings are used in conjunction with other components, such as torque motors, resolvers and encoders. Electrical slip-rings must be designed to be located either on the platform axis of rotation, or be designed with an open bore which locates the electrical contacts off-axis. Hence, the designations “on-axis” and “off-axis” slip-rings, respectively.
The diameters of slip-rings may range from a fraction of an inch to multiple feet, and the relative angular speed (ω) between the rotor and stator may vary from one revolution per day to as much as 20,000 revolutions per minute (rpm). In all of these various applications, the electrical contacts between the rotor and stator should: (1) be able to transfer power and/or signal(s) without interruption at high relative surface speeds, (2) have long wear life, (3) have low electrical noise, and (4) be of a physical size that allows multiple circuits to be packaged in a minimum volume.
Proper management of the electrical and mechanical contact physics between the brush assembly and the rotor allows demanding requirements to be met. For example, if the application is an off-axis slip-ring that allows an x-ray tube in a CT scan gantry to rotate about the patient's body, the electrical contacts must be designed to carry about 100-200 amps (with possible surges of hundreds of amps), to operate at surface speeds on the order of 15 meters per second (m/sec), to last for 100 million revolutions, and to occupy a minimal volume within the gantry. In order to meet the 100 million revolution requirement for a device that is about six feet [1.8288 meters (“m”)] in diameter, the brush force (i.e., the force with which the brush tips are urged against the rotor) must be low to minimize frictional heating and yet maintain a large number of contact points between the brush and rotor ring to achieve the required current density.
There has been a renewed interest in the use of fibrous metal brushes in recent years. Metal fiber brushes have the capability of providing higher current densities, of having lower electrical noise, and of having longer life while operating at higher surface speeds. Each of these parameters is related to more points of contact between brush and rotor ring than with composite brushes, less force per fiber, and less frictional heating. The area of contact between the fiber tips and a rotor ring is known as the “interfacial” area of contact. It is known that the actual area of contact between the face of a composite brush and a rotor is much less than its geometric area. Hence, the reason for sub-dividing brushes into elements which, in some cases, are individual small-diameter fibers.
The tribological properties of electrical contacts and the right choice of lubricant to meet the requirements of the application are extremely important. For example, if the contacts are to be used in a space application, the lubricant must not only meet all of the requirements of a ground-based application, but must also have a low vapor pressure as well. If the contacts have a long-life requirement, then dust, wear debris and other contaminants may accumulate in the contact zone and create problems with life and signal transfer.
Accordingly, it would be highly desirable to provide improved electrical contacts for transmitting electrical power and/or signal(s) between a rotor and a stator.
It would also be highly desirable to provide improved fiber brush assemblies for use in such slip-rings.
It would also be highly desirable to provide improved slip-rings that employ FOT technology, and that allow a brush assembly to have a longer life at higher rotor surface speeds and at lower cost than with current FOT technology.